Yttrium aluminum garnet (YAG), Y3Al5O12, ceramics are of interest for use in the manufacture of discharge vessels of HID lamps from both an optical and mechanical standpoint. Unlike the polycrystalline alumina (PCA) ceramics which are prevalent in commercial HID lamps, polycrystalline YAG ceramics have no bi-refringence effect at the grain boundaries and therefore can have a much higher in-line transmittance than PCA. The higher in-line transmittance makes polycrystalline YAG ceramics desirable for short-arc, focused-beam applications such as automotive headlamps and photo-optical lamps. In addition, yttrium aluminum garnet has a cubic symmetry and exhibits isotropic thermal expansion. The isotropic thermal expansion means that there are no residual stresses in the YAG ceramics whereas PCA by comparison has expansion-anisotropy induced residual stresses at the grain boundaries. Also, the elastic constant (Young's modulus) of YAG is about the same as PCA and YAG has a high resistance to creep deformation at elevated temperatures. As a result, YAG ceramics possess a higher mechanical strength (at a given flaw size) than PCA ceramics, especially at high temperatures (>1000° C.).
In order to achieve transparency, dopants are generally required to aid the sintering of compacts of commercially available YAG powders. Of course, it would be best if no sintering aids were used. However, without a sintering aid, it is necessary to use highly active starting powders which are costly to synthesize. The highly reactive powders which have been described in the prior art have a surface area on the order of 15 m2/g and an average particle size less than 0.5 μm. See, e.g., Japanese Patent Publication No. 2001-158660A, European Patent Application EP 0 926 106 A1, and Japan Cer. Soc. Journal, 98 [3]285-91 (1990). By contrast, commercially available powders generally have a low surface area, less than 5 m2/g, and a average particle size greater than or equal to 1 μm. More particularly, the surface area ranges from 3.6 to 4.8 m2/g and the average particle size from 1 to 3 μm.
For HID lamp applications, the selection of an appropriate sintering aid for making a transparent discharge vessel involves special considerations such as corrosion resistance and stoichiometry stability. The corrosion resistance of YAG and PCA ceramics is affected (1) by the presence of secondary grain boundary phases derived from the dopants, and (2) by the segregation of the dopants to the grain boundaries. For example, it is widely known that an SiO2 additive in PCA would cause the formation of grain-boundary silicate phases which are subject to attack by the corrosive fill materials used in conventional metal halide and high-pressure sodium lamps.
The stoichiometry stability of YAG in a reducing environment, or under ultraviolet radiation, is related to the formation of atomic defects—oxygen vacancies and interstitials—that affect the discoloration behavior. For example, it has been observed that the optical absorption in YAG is a strong function of the oxygen partial pressure in the atmosphere. Very reducing environments typically induce the formation of more oxygen vacancies in YAG. Therefore, YAG ceramics are typically sintered in either vacuum or wet hydrogen. If dry hydrogen is used as the sintering atmosphere, the YAG ceramics become grayish from the oxygen vacancies which are produced. Similarly, lamp tests have shown that YAG discharge vessels become slightly grayish after about 9000 hours of operation in the reducing environment of the lamp.
The stoichiometric stability of these ceramics is also related to the dopants used. For example, MgO-doped YAG that has been sintered in wet hydrogen turns pink-red upon a post-sinter air firing at 1000-1350° C. The pink-red discoloration of the MgO-doped YAG is indicative of the general stoichiometric stability and in particular of the sensitivity of YAG to the partial pressure of oxygen. Thus, the appearance or absence of the pink-red discoloration following the post-sinter air firing can be used as a test to predict performance of a YAG ceramic over the operating life of an HID lamp.